1. Field of the Invention
This invention relates to a dielectric filter comprising a plurality of dielectric resonators of which adjacent ones are connected to each other electromagnetically or via a coupling element. More particularly, the present invention relates to a band-pass type dielectric filter having a pole in an attenuation region.
2. Description of the Prior Art
In a band-pass filter, it is sometimes requested by a user that an excellent frequency attenuation should be obtained in a certain region that is separated from the center frequency by a certain degree. To accomplish the aforesaid request, in a dielectric filter comprising a plurality of resonators, whether cavity or dielectric type, of which adjacent ones are connected to each other electromagnetically or via a coupling element, one method is to increase the number of stages in the resonator. Another method, according to which no increase in the number of resonator stages is required, is to skip resonators of one or more stages and to directly connect electromagnetically the resonators on opposite sides of the skipped resonators. By this method, poles P1 and P2 appear in an attenuation region as shown by a solid line in FIG. 17 and the skirt portion of the dielectric filter characteristics becomes very steep. Consequently, the frequency attenuation becomes greater than that of the predetermined level, in a region that is separated from the center frequency by the predetermined frequency, thereby satisfying the request described above. In FIG. 17, a broken line shows the frequency characteristics of a dielectric filter having the same number of resonator stages but with no pole.
Technology for forming a pole in an attenuation region of the dielectric filter characteristics (hereinafter referred to as "polarization") as mentioned above is known and used in a cavity resonator filter or a semi-coaxial filter, but has not yet been taught or suggested to use in a dielectric filter.